This invention relates to a microporous or ultrafiltration membrane having a hydrophilic surface and formed from an interpenetrating network of a hydrophobic polymer and a hydrophilic cross linked monomeric composition and to the process for forming the membrane.
In many applications of filtration technology, it is desirable to utilize a membrane filter which is mechanically strong, is thermally stable, is relatively inert chemically and is insoluble in most organic solvents. Often, it is desirable that the membrane has surface properties which are radically different from, and sometimes incompatible with, the bulk properties of the membrane. Desirable surface properties include wettability, low protein adsorbing tendency, thromboresistivity, controlled ion exchange capacity and controlled surface chemical reactivity.
Conventional methodology presently used to achieve the duality of function of bulk properties which differ from the surface properties is to coat a preformed membrane having the desired bulk properties with an oligomer or polymer having the desired surface properties. Typical coating materials include surfactants and water soluble polymers such as polyvinylpyrrolidone. This approach has disadvantages, particularly because the coating reduces flux by reducing pore size; flux reduction is especially severe for small (&lt;0.1 u) pore membranes.
It also has been proposed to utilize graft polymerization techniques to modify the surface characteristics of a polymeric substrate. Typical examples of graft polymerization are shown for example in U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573. It is difficult to utilize presently available graft polymerization techniques to modify the surface properties of the porous membrane. This is because it is difficult to modify the entire surface of the membrane including the surfaces within the pores while avoiding significant pore blockage and while retaining membrane porosity.
It has been proposed in U.S. Pat. No. 4,618,533 to form a porous membrane having a porous membrane substrate to which is directly coated a cross-linked polymer formed from a monomer polymerized with a free radical initiator in situ on the substrate. The resulting composite membrane has essentially the same porous configuration as the porous substrate. It is disclosed that the presence of a polymerization initiator and a cross-linking agent are necessary in order to effect the desired polymerization and cross-linking in situ and thereby to obtain the desired porous configuration of the membrane product, i.e., little or no blockage of the pores, because the pores are large.
U.S. Pat. No. 4,787,976 discloses a permeable membrane that does not adsorb protein and is formed from a solution of a hydrophilic urethane prepolymer and a soluble polymer cast on a substrate. The cast film then is immersed in a coagulation bath such as an aqueous bath which effects polymerization of the prepolymer as well as the formation of pores in the film.
U.S. Pat. No. 4,119,581 discloses a method for producing an ion-exchange non-porous membrane from a solution of a thermoplastic polymer and monomers which, when exposed to heat or gamma radiation, become polymerized to form an interpenetrating network of polymer chains. The monomers used to form the ion exchange capacity of the membrane are a diamine and a dihalide. They undergo the Menshutkin reaction to produce inherently ionic polymers.
U.S. Pat. No. 4,302,334 discloses a process for making a microporous membrane from a casting solution of a hydrophobic polyvinylidene fluoride and a vinyl acetate polymer. The resulting interpenetrating polymer network is cast on a substrate in the usual way and coagulated to form a porous membrane. The polyvinyl acetate is then hydrolyzed to polyvinyl alcohol, a hydrophilic polymer.
U.S. Pat. No. 4,012,324 discloses a method for making porous membranes from a solution of a method for making matrix polymeric mixture and a cross-linking agent, e.g., an epoxy. Cross-linking is effected by heating.
The prior art discloses a variety of ways to produce porous hydrophilic membranes from materials that are primarily hydrophobic. None discuss a two step process for producing hydrophilic porous membranes by polymerizing a hydrophilic monomer in a solution of a hydrophobic polymer, subsequently casting a membrane from the solution; and then annealing to produce a hydrophilic membrane.